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Oxidation of Chromium(III) to (VI) by Manganese Oxides

^a Dep. of Soil and Crop Sciences, Texas A&M Univ., College Station, TX 77843-2474
^b Dep. of Chemistry, P.O. Box 30012, Texas A&M Univ., College Station, TX 77842-3012

View larger version (30K): [in a new window]   Fig. 1. X-ray diffraction patterns of the coarse clay-size (0.2–2 µm) Mn oxides (synthetic pyrolusite and natural lithiophorite, todorokite, and birnessite). The unit of d-spaces in the figures is angstroms (Å). The XRD peaks marked with * stand for silicates.

View larger version (18K): [in a new window]   Fig. 2. Infrared curves of the coarse clay-size Mn oxides (%T equals % transmission).

View larger version (100K): [in a new window]   Fig. 3. Transmission electron micrographs of (a) todorokite (b) birnessite, (c) lithiophorite, and (d) pyrolusite.

View larger version (46K): [in a new window]   Fig. 4. Bar graphs of relative binding energy of Mn 2p3/2 for todorokite (T), birnessite (B), lithiophorite (L), and pyrolusite (P).

View larger version (26K): [in a new window]   Fig. 5. Binding energy curves O 1s electrons in the four Mn oxides. The abbreviations are todorokite (T), birnessite (B), lithiophorite (L), and pyrolusite (P).

View larger version (30K): [in a new window]   Fig. 6. Kinetics of Cr oxidation by the coarse clay-size Mn oxides at pH 4. The oxidation tests were conducted at (a) 200 µM and at (b) 400 µM of initial Cr(III) concentration. Data were plotted according to the first-order rate equation (Sparks, 1989). Concentration: Kt = Cr(III) at time t; K= Cr(III) at equilibrium.

View larger version (31K): [in a new window]   Fig. 7. Chromium oxidation by four Mn oxides in 12 h at pH 4 and 7. The four Mn oxides are abbreviated as follows: todorokite (T), birnessite (B), lithiophorite (L), and pyrolusite (P).